feat: min/max participants — shared ParticipantLimits component

- New ParticipantLimits component in FormField.tsx (reusable)
- Used in both Open Day settings and MC editor — identical layout
- Open Day: event-level min/max (DB migration 15)
- MC: per-event min/max (JSON fields)
- Public: waiting list when full, spots counter, amber success modal

src/lib/db.ts

@@ -265,6 +265,16 @@ const migrations: Migration[] = [
       }
     },
   },
+  {
+    version: 15,
+    name: "add_max_participants_to_open_day_events",
+    up: (db) => {
+      const cols = db.prepare("PRAGMA table_info(open_day_events)").all() as { name: string }[];
+      if (!cols.some((c) => c.name === "max_participants")) {
+        db.exec("ALTER TABLE open_day_events ADD COLUMN max_participants INTEGER NOT NULL DEFAULT 0");
+      }
+    },
+  },
 ];
 
 function runMigrations(db: Database.Database) {
@@ -958,6 +968,7 @@ interface OpenDayEventRow {
   discount_price: number;
   discount_threshold: number;
   min_bookings: number;
+  max_participants: number;
   active: number;
   created_at: string;
   updated_at: string;
@@ -972,6 +983,7 @@ export interface OpenDayEvent {
   discountPrice: number;
   discountThreshold: number;
   minBookings: number;
+  maxParticipants: number;
   active: boolean;
 }
 
@@ -1051,6 +1063,7 @@ function mapEventRow(r: OpenDayEventRow): OpenDayEvent {
     discountPrice: r.discount_price,
     discountThreshold: r.discount_threshold,
     minBookings: r.min_bookings,
+    maxParticipants: r.max_participants ?? 0,
     active: !!r.active,
   };
 }
@@ -1160,6 +1173,7 @@ export function updateOpenDayEvent(
     discountPrice: number;
     discountThreshold: number;
     minBookings: number;
+    maxParticipants: number;
     active: boolean;
   }>
 ): void {
@@ -1173,6 +1187,7 @@ export function updateOpenDayEvent(
   if (data.discountPrice !== undefined) { sets.push("discount_price = ?"); vals.push(data.discountPrice); }
   if (data.discountThreshold !== undefined) { sets.push("discount_threshold = ?"); vals.push(data.discountThreshold); }
   if (data.minBookings !== undefined) { sets.push("min_bookings = ?"); vals.push(data.minBookings); }
+  if (data.maxParticipants !== undefined) { sets.push("max_participants = ?"); vals.push(data.maxParticipants); }
   if (data.active !== undefined) { sets.push("active = ?"); vals.push(data.active ? 1 : 0); }
   if (sets.length === 0) return;
   sets.push("updated_at = datetime('now')");
@@ -1204,6 +1219,17 @@ export function addOpenDayClass(
   return result.lastInsertRowid as number;
 }
 
+export function getOpenDayClassById(classId: number): OpenDayClass | null {
+  const db = getDb();
+  const row = db.prepare(
+    `SELECT c.*, COALESCE(b.cnt, 0) as booking_count
+     FROM open_day_classes c
+     LEFT JOIN (SELECT class_id, COUNT(*) as cnt FROM open_day_bookings GROUP BY class_id) b ON b.class_id = c.id
+     WHERE c.id = ?`
+  ).get(classId) as OpenDayClassRow | undefined;
+  return row ? mapClassRow(row) : null;
+}
+
 export function getOpenDayClasses(eventId: number): OpenDayClass[] {
   const db = getDb();
   const rows = db